1. Field of the Invention
The present invention relates to a technology for controlling a wiper apparatus for use in a vehicle, and more particularly to a technology advantageously applied to a wiper apparatus having a wiper blade driven by a motor.
2. Related Art Statement
One of the essential requirement for a wiper apparatus is to drive the wiper blade accurately even if unexpected load is applied on the wiper blade.
Incidentally, there has been proposed a circuit configuration for controlling wiper blades in which a relay plate is employed in order that when the wiper switch is turned off during operation the wiper blade automatically stops at a predetermined position. FIG. 14 shows an example of a circuit arrangement applicable to a wiper apparatus. The left and right motors are respectively controlled in its stop operation so that when the wiper switch is turned off the wiper blade is moved to a predetermined angular position and then stopped. Here, the motor 51 is driven by a drive element 53 based on an instruction from the motor drive control unit 52. Turning On/OFF of the motor is executed by a wiper switch 54. Note that when the wiper switch 54 is turned off during operation the wiper blade automatically stops at a predetermined position owing to the function of a relay plate 55.
However, if the left or right wiper blades cause abnormal operation and the drive element 53 is turned off to halt the motor 51 during the drive range of the relay plate 65, there is a problem that the circuit becomes open and easily affected by an external force. That is, when a force is applied to a wiper blade or the like, electro-magnetic braking does not work due to the open state of the circuit. If the external force is greater than the motor static force, the motor cannot withstand the force but is rotated. Therefore, the wiper blade moves and interferes with the other wiper blade if snow, for example, falls on the wiper blade when the wiper blade is stopped to wait.
Incidentally, there are two types of controlling manners in wiper blades. One of them is a parallel type in which a pair of wiper blades are driven while keeping substantially equidistant position between the blades. The other of them is an opposing type in which the pair of wiper blades are controlled to move in an opposing fashion. That is, when one of the blades moves to the center of the front glass the other of the wiper blades also moves to the center of the front glass while one moves away from the center the other is also moves away from the center. In a wiper apparatus of the opposed wiping type, rotation centers of wiper arms are respectively positioned at left and right ends of the front glass and wiper blades move from both sides of the front glass toward the center thereof.
When the left and right wiper blades are driven by respective motors, the mechanism can suffer from asynchronous motions in both of the wiper blades. The asynchronous motion is caused from difference in motor drive characteristics between the left and right motors or the motor rotational rate variations due to lord imposed on the motors.
Moreover, it is general for the wiper apparatus of the opposing type to have an arrangement in which the wiping areas of the left and right wiper blades are normally overlapped at the center portion, or a lower return position of the front glass, so as to enlarge the whole wiping area. If the wiper apparatus is made to have such arrangement, the apparatus is more stringently required to avoid the interference in motion between the pair of blades due to the asynchronous motion of the blades. Accordingly, the motors employed in the apparatus thus arranged are more stringently requested to have equal characteristics and the load variation shall be subjected to more stringent control for eliminating the interference.
In order to avoid the interference in the blade motion in the wiper apparatus set forth above, it is a possible way to control both of the wiper blades independently based on each of the angular position taken by each of the wiper blades. In order to detect the angular position currently taken by the wiper blade, it is a possible way that the driving motor for the wiper blade is coupled to a rotary encoder and the rotary encoder is arranged to generate a number of pulses which is in proportion to the traveling rotation angle of the motor shaft. The number of pulses generated from the rotary encoder is counted and the counted number is regarded as data indicative of the angular position taken by the wiper blade coupled to the motor.
However, the wiper blade employed in the wiper apparatus can also suffer from unexpected external force such as wind, rain, snow or the like, and if such external force is applied to the wiper blade, making the wiper blade move in the undesired direction, the counted number of pulses cannot represent an angular position which is actually taken by the wiper blade. If correspondence between the pulse count and angular position taken by the wiper blade is not assured, it becomes impossible to control the wiper motion accurately.